Transistor Having Elevated Drain Finger Termination

ABSTRACT

According to an exemplary implementation, a transistor includes drain finger electrodes interdigitated with source finger electrodes. The transistor also includes a current conduction path in a semiconductor substrate between the drain finger electrodes and the source finger electrodes. At least one of the drain finger electrodes has a drain finger electrode end and a drain finger electrode main body, where the drain finger electrode main body is non-coplaner with at least a portion of the drain finger electrode end. The transistor may also include a dielectric material situated between at least a portion of the drain finger electrode end and the semiconductor substrate. The dielectric material can be an increasing thickness dielectric material. The dielectric material can thus elevate the drain finger electrode end over the semiconductor substrate. Further, the drain finger electrode end can have an increased radius of curvature.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with Government support under Contract No.DE-AR0000016 awarded by Advanced Research Projects Agency-Energy(ARPA-E). The Government has certain rights in this invention.

BACKGROUND

The present application claims the benefit of and priority to U.S.Provisional Patent Application Ser. No. 61/674,553, filed on Jul. 23,2012 and entitled “Elevated Drain Finger Termination,” and is acontinuation-in-part of U.S. patent application Ser. No. 13/749,477,filed on Jan. 24, 2013 and entitled “Transistor Having IncreasedBreakdown Voltage,” which itself claims the benefit of and priority toU.S. Provisional Patent application Ser. No. 61/600,469, filed on Feb.17, 2012 and entitled “Drain Termination for Transistor with ImprovedBreakdown Voltage.” The disclosures of the above applications are herebyincorporated fully by reference into the present application.

I. Definitions

As used herein, the phrase “group III-V” refers to a compoundsemiconductor including at least one group III element and at least onegroup V element. By way of example, a group Ill-V semiconductor may takethe form of a HI-Nitride semiconductor. “III-Nitride”, or “III-N”,refers to a compound semiconductor that includes nitrogen and at leastone group III element such as aluminum (Al), gallium (Ga), indium (In),and boron (B), and including but not limited to any of its alloys, suchas aluminum gallium nitride (Al_(x)Ga_((1-x))N), indium gallium nitride(In_(y)Ga_(((1-y))N), aluminum indium gallium nitride(Al_(x)In_(y)Ga_((1-x-y))N), gallium arsenide phosphide nitride(GaAs_(a)P_(b)N_((1-a-b))), aluminum indium gallium arsenide phosphidenitride (Al_(x)In_(y)Ga_((1-x-y))As_(a)P_(b)N_((1-a-b))), for example.III-Nitride also refers generally to any polarity including but notlimited to Ga-polar, N-polar, semi-polar, or non-polar crystalorientations. A III-Nitride material may also include either theWurtzitic, Zincblende, or mixed polytypes, and may includesingle-crystal, monocrystalline, polycrystalline, or amorphousstructures. Gallium nitride or GaN, as used herein, refers to aIII-Nitride compound semiconductor wherein the group III element orelements include some or a substantial amount of gallium, but may alsoinclude other group III elements in addition to gallium. A group III-Vor a GaN transistor may also refer to a composite high voltageenhancement mode transistor that is formed by connecting the group III-Vor the GaN transistor in cascode with a lower voltage group IVtransistor.

In addition, as used herein, the phrase “group IV” refers to asemiconductor that includes at least one group IV element such assilicon (Si), germanium (Ge), and carbon (C), and may also includecompound semiconductors such as silicon germanium (SiGe) and siliconcarbide (SiC), for example. Group IV also refers to semiconductormaterials which include more than one layer of group IV elements, ordoping of group IV elements to produce strained group IV materials, andmay also include group IV based composite substrates such as silicon oninsulator (SOI), separation by implantation of oxygen (SIMOX) processsubstrates, and silicon on sapphire (SOS), for example.

II. Background Art

Group III-V heterostructure field-effect transistors (group III-VHFETs), such as group III-V high electron mobility transistors (groupIII-V HEMTs), are often utilized in high power switching applications.For example, III-Nitride HEMTs may be utilized to provide switchingand/or amplification functions.

Group III-V HFETs advantageously allow for power transistors using alateral conduction topology in which drain, source, and gate electrodesare formed on one side of a semiconductor wafer or die. In a typicallateral transistor layout, for example, drain and source fingerelectrodes coupled to respective drain and source pads may beinterdigitated with a gate region located between the source and drain.As the power requirement for such transistors continues to increase, thetransistors are fabricated with a higher unit cell density. However, dueto high termination electric fields typically present at the drainfinger electrode ends, the gate-to-drain spacing at the drain fingerelectrode ends of higher density power transistors may be insufficientto reliably sustain a high breakdown voltage.

SUMMARY

A transistor having elevated drain finger termination such that at leasta portion thereof is non-coplanar with the drain finger electrode mainbody, substantially as shown in and/or described in connection with atleast one of the figures, and as set forth more completely in theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 presents a top plan view of a transistor.

FIG. 2 presents a top plan view of an exemplary transistor having adrain termination enabling increased breakdown voltage, in accordancewith an implementation of the present disclosure.

FIG. 3A presents a top plan view of an exemplary drain terminationenabling increased breakdown voltage, corresponding in general to theimplementation shown in FIG. 2.

FIG. 3B presents a top plan view of an exemplary drain terminationenabling increased breakdown voltage, corresponding in general to theimplementation shown in FIG. 2.

FIG. 4 presents a top plan view of an exemplary drain terminationenabling increased breakdown voltage, corresponding in general to theimplementation shown in FIG. 2.

FIG. 5 presents a top plan view of an exemplary drain terminationincluding a tapered drain finger electrode enabling increased breakdownvoltage, in accordance with an implementation of the present disclosure.

FIG. 6A presents a top plan view of an exemplary transistor having adrain termination enabling increased breakdown voltage, in accordancewith an implementation of the present disclosure.

FIG. 6B presents a cross-sectional view of an exemplary transistor, inaccordance with an implementation of the present disclosure.

FIG. 6C presents a cross-sectional view of an exemplary transistorhaving a drain finger electrode main body which is non-coplaner with thedrain finger termination, in accordance with an implementation of thepresent disclosure.

DETAILED DESCRIPTION

The following description contains specific information pertaining toimplementations in the present disclosure. The drawings in the presentapplication and their accompanying detailed description are directed tomerely exemplary implementations. Unless noted otherwise, like orcorresponding elements among the figures may be indicated by like orcorresponding reference numerals. Moreover, the drawings andillustrations in the present application are generally not to scale, andare not intended to correspond to actual relative dimensions.

FIG. 1 presents a top plan view of a transistor. Transistor 100 hasactive surface 102 including source pad 104, drain pad 106, and gate pad108. Transistor 100 also includes gate region 109 adjoining gate pad108, source finger electrodes 111, drain finger electrodes 121, andfield dielectric 107 isolating source finger electrodes 111 and drainfinger electrodes 121 from gate region 109. Source finger electrodes 111are electrically coupled to source pad 104, and drain finger electrodes121 are electrically coupled to drain pad 106. It is noted that drainfinger electrodes 121 are terminated adjacent source pad 104 by beingsimply rounded off to form semicircular drain finger electrode ends 125.It is further noted that the electrical coupling of source fingerelectrodes 111 and drain finger electrodes 121 to respective source pad104 and drain pad 106 is shown by dashed lines because those connectionsmay be formed using one or more additional metal layers not shown inFIG. 1.

Source finger electrodes 111 and drain finger electrodes 121 aresituated in gate region 109 and are implemented in an interdigitatedpattern such that gate region 109 surrounds source finger electrodes 111and drain finger electrodes 121, and is disposed between theinterdigitated source finger electrodes 111 and drain finger electrodes121. As further shown in FIG. 1, transistor 100 has source-to-drainhalf-pitch 135, i.e., the distance separating center line 131 of sourcefinger electrode 111 from center line 132 of drain finger electrode 121.

Transistor 100 has an interdigitated layout commonly utilized toimplement lateral power transistors. As shown in FIG. 1, source-to-drainhalf-pitch 135 is substantially constant along the entirety of drainfinger electrodes 121. However, due to the high termination electricfields, which may develop under high voltage operation, particularly atsemicircular drain finger electrode ends 125, the robustness oftransistor 100 may be compromised. As a result, transistor 100 may breakdown, become unstable, and/or fail catastrophically under high voltageoperation. In accordance with an implementation of the presentdisclosure, various approaches are described, which can be utilizedalone or in any combination to ease the high termination electricfields, amongst other advantages.

FIG. 2 presents a top plan view of an exemplary transistor having adrain termination enabling increased breakdown voltage, in accordancewith an implementation of the present disclosure. FIG. 2 showstransistor 200 fabricated in active surface 202 of semiconductor body201. Transistor 200 includes drain finger electrodes 220 interdigitatedwith source finger electrodes 210. Transistor 200 may be a high voltagetransistor.

It is noted that as used herein, the feature referred to as a“transistor” may correspond more generally to a variety of specifictransistor or other semiconductor device implementations. In oneimplementation, for example, transistor 200 may take the form of afield-effect transistor (FET). For instance, transistor 200 may be aninsulated-gate FET (IGFET) or a heterostructure FET (HFET). In oneimplementation, transistor 200 may take the form of ametal-insulator-semiconductor FET (MISFET), such as ametal-oxide-semiconductor FET (MOSFET). In other implementations,transistor 200 may take the form of a schottky gated transistor, or aP-N junction gated transistor or (MET). When implemented as an HFET,transistor 200 may be a high-electron-mobility transistor (HEMT)incorporating a two-dimensional electron gas (2DEG). In yet anotherimplementation, transistor 200 may take the form of a Schottky diode(not shown in FIG. 2), for example through replacement of drain fingerelectrodes 220 with anode electrodes, replacement of the source fingerelectrodes 210 with cathode electrodes, and without the controlling gateelectrode of FIG. 2.

Referring to the exemplary implementation shown by FIG. 2, semiconductorbody 201 of transistor 200 may be configured to provide a group III-VHFET, such as a III-Nitride HEMT. III-Nitride materials include galliumnitride (GaN) and/or its alloys, such as aluminum gallium nitride(AlGaN), indium gallium nitride (InGaN), and aluminum indium galliumnitride (AlInGaN). These materials are semiconductor compounds that havea relatively wide, direct bandgap and strong piezoelectricpolarizations, and can enable high breakdown fields, and the creation of2DEGs. As a result, III-Nitride materials such as GaN are used in manymicroelectronic applications in which high power density and highefficiency switching are required.

Active surface 202 of transistor 200 includes source pad 204, drain pad206, and gate pad 208. In addition to drain finger electrodes 220 andsource finger electrodes 210, transistor 200 also includes gate region209, and field dielectric 207. Source finger electrodes 210 areelectrically coupled to source pad 204, drain finger electrodes 220 areelectrically coupled to drain pad 206, and gate region 209 iselectrically coupled to gate pad 208. It is noted that the electricalcoupling of source finger electrodes 210 to source pad 204 and that ofgate region 209 to gate pad 208 is shown by dashed lines because thoseconnections may be formed using one or more additional metal layers notshown in FIG. 2.

It is noted that in some implementations, either or both of source pad204 and drain pad 206 may be on the same plane as source fingerelectrodes 210 and drain finger electrodes 220. However, in otherimplementations, source pad 204 and/or drain pad 206 may be on adifferent plane than that occupied by source finger electrodes 210and/or drain finger electrode 220. For example, source pad 204 and/ordrain pad 206 may be formed on a backside of the device or substrate andelectrically connected to their respectively corresponding fingerelectrodes using electrical through-substrate vias. Such transistordesigns with ohmic pad (source and/or drain) construction on otherplanes are described in U.S. Pat. No. 7,233,028, entitled “GalliumNitride Material Devices and Methods of Forming the Same,” and issued onJun. 19, 2007; and in U.S. Pat. No. 7,915,645, entitled “MonolithicVertically Integrated Composite Group III-V and Group IV SemiconductorDevice and Method for Fabricating Same,” and issued on Mar. 29, 2011.The entire disclosures of U.S. Pat. No. 7,233,028 and U.S. Pat. No.7,915,645 are hereby incorporated fully by reference into the presentapplication.

As shown in FIG. 2, source finger electrodes 210 and drain fingerelectrodes 220 may be implemented in an interdigitated pattern such thatgate region 209 surrounds source finger electrodes 210 and is betweeninterdigitated source finger electrodes 210 and drain finger electrodes220. Source finger electrodes 210 have length 212, and drain fingerelectrodes 220 have length 222. Also shown in FIG. 2 is source-to-drainhalf-pitch 235 of transistor 200, i.e., the distance separating centerline 231 of source finger electrode 210 from center line 232 of drainfinger electrode 220.

Source finger electrodes 210 have source finger electrode main bodies214 coupled to source pad 204, and source finger electrode ends 216adjacent drain pad 206. Drain finger electrodes 220 have drain fingerelectrode main bodies 224 coupled to drain pad 206, and curved drainfinger electrode ends or tips 226 adjacent source pad 204. As shown inFIG. 2, source-to-drain half-pitch 235 of transistor 200 issubstantially constant from source finger electrode main bodies 214 tosource finger electrode ends 216. As further shown in FIG. 2, curveddrain finger electrode ends 226 are modified in order reduce theelectric field at the tips of drain finger electrodes 220.

In at least one exemplary implementation, curved drain finger electrodeends 226 have an increased radius of curvature, such as (r1) 236, forexample, resulting in increased width 246 of curved drain fingerelectrode ends 226 relative to width 254 of drain finger electrode 220.It is noted that as used herein, width 254 is defined as the width ofdrain finger electrode 220 just before the transition to curved drainfinger electrode end 226. As also shown in FIG. 2, radius of curvature(r2) 241 of gate region 209 surrounding curved drain finger electrodeend 226 is also increased, which increases the gate-to-drain spacing(d2) 257 in the vicinity of curved drain finger electrode ends 226compared to the gate-to-drain spacing (d1) 267, further reducing theelectric field at curved drain finger electrode ends 226. It is notedthat for the purposes of the present application, when curved drainfinger electrode ends 226 are described as having an increased radius ofcurvature, that characterization can apply to either one, or both, of(r1) 236 and (r2) 241.

In some implementations, as shown in FIG. 2, increased radius ofcurvature (r1) 236 of curved drain finger electrode ends 226 may causecurved drain finger electrode ends 226 to have a generally ellipsoidalshape as discussed in more detail in reference to FIGS. 3A, 3B, and 4,below. One advantage of implementing curved drain finger electrode ends226 as substantial ellipsoids is that such a shape can aid in reducingor relaxing the high termination electrical fields that can otherwiseexist at semicircular drain finger electrode ends 125 of transistor 100,shown in FIG. 1. As a result, increased radius of curvature (r1) 236and/or increased radius of curvature (r2) 241, as well as the subsequentincrease in gate-to-drain spacing (d2) 257 at curved drain fingerelectrode ends 226, reduces the termination electric field at curveddrain finger electrode ends 226 so as to achieve an increased breakdownvoltage and reliability for transistor 200.

In some implementations, curved drain finger electrode ends 226 areextended beyond source finger electrode main bodies 214 instead of or inaddition to providing curved drain finger electrode ends 226 having anincreased radius of curvature. As depicted in FIG. 2, curved drainfinger electrode ends 226 are situated closer to source pad 204 thansource finger electrode main bodies 214. In doing so, drain fingerelectrodes 220 may be longer than source finger electrodes 210, asshown. Extension of curved drain finger electrode ends 226 beyond sourcefinger electrode main bodies 214 can advantageously achieve an increasedbreakdown voltage for transistor 200 without increasing source-to-drainhalf-pitch 235 along length 212 of source finger electrodes 210.Consequently, the exemplary transistor layout shown in FIG. 2 can beimplemented so as to allow for a tighter source-to-drain half-pitchwhile achieving increased breakdown voltage, thereby providing a lowON-resistance and improved robustness for transistor 200, concurrently.

It is noted that although the implementation shown in FIG. 2 depictsdrain finger electrodes 220 as having increased radius of curvature (r1)236, increased radius of curvature (r2) 241, and increased width 246 atcurved drain finger electrode ends 226, as well as having curved drainfinger electrode ends 226 extending beyond source finger electrode mainbodies 214, that representation is merely exemplary. In otherimplementations, drain finger electrodes 220 may not have all of thosecharacteristics concurrently.

In one implementation, curved drain finger electrode ends 226 may extendbeyond source finger electrode main bodies 214 but not have increasedradius of curvature (r1) 236, increased radius of curvature (r2) 241, orincreased width 246. Alternatively, according to another implementation,curved drain finger electrode ends 226 may have increased radius ofcurvature (r1) 236, increased width 246, and/or increased radius ofcurvature (r2) 241, but may not extend beyond source finger electrodemain bodies 214. Thus, according to a variety of possibleimplementations, drain finger electrodes 220 may have length 222 greaterthan length 212 of source finger electrodes 210, and/or may have curveddrain finger electrode ends 226 extending beyond source finger electrodemain bodies 214, and/or may have increased radius of curvature (r1) 236,increased width 246, and/or increased radius of curvature (r2) 241 atcurved drain finger electrode ends 226.

Some of the features discussed in conjunction with FIG. 2 will now befurther described by reference to FIGS. 3A, 3B, and 4. FIGS. 3A, 3B, and4 present respective top plan views of exemplary drain terminationshaving a decreased electric field at the ends or tips of the drainfinger electrodes. As further described below, such a decreased electricfield can result from increasing a radius of curvature of thegate-to-drain spacing between an edge of a curved drain finger electrodeend and a boundary of a gate region surrounding the curved drain fingerelectrode end, thereby allowing for an increased breakdown voltage and amore robust and reliable transistor. It is noted that the exemplaryimplementations shown in FIGS. 3A, 3B, and 4 corresponding in general tothe implementation shown in FIG. 2.

FIG. 3A presents a top plan view of an exemplary drain terminationenabling increased breakdown voltage, corresponding in general to theimplementation shown in FIG. 2. FIG. 3A shows a portion of gate region309 surrounding drain finger electrode 320. Also shown in FIG. 3A aresource finger electrodes 310 with which drain finger electrode 320 isinterdigitated, and field dielectric 307. Gate region 309, drain fingerelectrode 320, source finger electrodes 310, and field dielectric 307correspond respectively to gate region 209, drain finger electrodes 220,source finger electrodes 210, and field dielectric 207, in FIG. 2.

FIG. 3A shows the relative width of drain finger electrode 320 comparedto source finger electrodes 310, as well as the smooth transitions atthe curved end of drain finger electrode 320. It is noted that there areno sharp discontinuities in the curvature of the drain finger electrodeend. In addition, and as further shown by FIG. 3A, there are no sharpdiscontinuities at the boundary of field dielectric 307 with gate region309, which also aids in reducing the electric fields at the end of drainfinger electrode 320.

Continuing to FIG. 3B, FIG. 3B presents a top plan view of an exemplarydrain termination enabling increased breakdown voltage, corresponding ingeneral to the implementation shown in FIG. 2. FIG. 3B shows gate region309 surrounding drain finger electrode 320. Drain finger electrode 320has width 354, and also includes drain finger electrode main body 324having width 334. In addition, FIG. 3B shows curved drain fingerelectrode end 326 having increased radius of curvature (r1) 336 relativeto the radius of curvature of semicircular drain finger electrode ends125 in transistor 100 of FIG. 1. That is to say, in contrast totransistor 100 having semicircular drain finger ends 125, radius ofcurvature (r1) 336 of curved drain finger end 326 is greater than aradius of curvature of a circle having a diameter equal to width 354.

Also shown in FIG. 3B is radius of curvature (r2) 341 of the gateelectrode region surrounding the drain finger electrode end 326, radiusof curvature (r3) 371 of the smooth transition connecting curved drainfinger electrode end 326 to drain finger electrode 320, and radius ofcurvature (r4) 381 of the smooth transition connecting gate region 309surrounding curved drain finger electrode end 326 to the gate regionsurrounding drain finger electrode 320. FIG. 3B further depictsgate-to-drain spacing (d1) 367, gate-to-drain spacing (d2) 357, andlength 322 of drain finger electrode 320. Gate-to-drain spacing (d1)367, gate-to-drain spacing (d2) 357, length 322, and width 354,correspond respectively to gate-to-drain spacing (d1) 267, gate-to-drainspacing (d2) 257, length 222, and width 254, in FIG. 2. Moreover, curveddrain finger electrode end 326, radius of curvature (r1) 336, and radiusof curvature (r2) 341, in FIG. 3B, correspond respectively to curveddrain finger electrode ends 226, radius of curvature (r1) 236, andradius of curvature (r2) 241, in FIG. 2.

In certain implementations, r1<r2<r4. Moreover, gate-to-drain spacing(d1) 367 is shown as being less than gate-to-drain spacing (d2) 357. Byforming the gate-to-drain spacing such that d1<d2, the reduced electricfield at curved drain finger electrode end 326, in combination with theellipsoidal shape of curved drain finger electrode end 326 and thesmooth curvatures created by r1, r2, r3, and r4, allows for theformation of a more robust drain finger electrode termination. It isnoted that for the purposes of the present application, when curveddrain finger electrode end 326 is described as having an increasedradius of curvature, that characterization can apply to any one of (r1)336, (r2) 341, (r3) 371, or (r4) 381, as well as to any combination of(r1) 336, (r2) 341, (r3) 371, and (r4) 381.

Continuing to FIG. 4, FIG. 4 presents a top plan view of an exemplarydrain termination enabling increased breakdown voltage, corresponding ingeneral to the implementation shown in FIG. 2. FIG. 4 shows gate region409 surrounding drain finger electrode 420. Drain finger electrode 420has width 454 and includes drain finger electrode main body 424 havingwidth 434. In addition, drain finger electrode 420 has curved drainfinger electrode end 426 with increased width 446 relative to width 454of drain finger electrode 420. Also shown in FIG. 4 are field dielectric407, radius of curvature (r1) 436, radius of curvature (r2) 441, radiusof curvature (r3) 471, radius of curvature (r4) 481, gate-to-drainspacing (d1) 467, and gate-to-drain spacing (d2) 457, as well as minoraxis 442 and major axis 443 of curved drain finger electrode end 426.

Gate region 409, field dielectric 407, and drain finger electrode 420correspond respectively to gate region 309, field dielectric 307, anddrain finger electrode 320, in FIGS. 3A and 3B. In addition, width 454,curved drain finger electrode end 426, and drain finger electrode mainbody 424 having width 434, correspond respectively to width 354, curveddrain finger electrode end 326, and drain finger electrode main body 324having width 334, in FIG. 3B. Moreover, gate-to-drain spacings (d1) 467and (d2) 457, and radii of curvature (r1) 436, (r2) 441, (r3) 471, and(r4) 481, in FIG. 4, correspond respectively to gate-to-drain spacings(d1) 367 and (d2) 357, and radii of curvature (r1) 236, (r2) 241, (r3)371, and (r4) 381, in FIG. 3B. It is further noted that increased width446 of curved drain finger electrode end 426, in FIG. 4, corresponds toincreased width 246 of curved drain finger electrode end 226, in FIG. 2.

As shown in FIG. 4, curved drain finger electrode end 426 has a shapedescribed by major axis 443 and minor axis 442. Major axis 443 is longerthan and substantially perpendicular to minor axis 442. As further shownin FIG. 4, major axis 443 is substantially aligned with drain fingerelectrode 420, and minor axis 442 is substantially perpendicular todrain finger electrode 420.

In some implementations, for example, curved drain finger electrode end426 may be substantially an ellipsoid having major axis 443substantially aligned with drain finger electrode 420. Curved drainfinger electrode end 426 taking the form of an ellipsoid with major axis443 and minor axis 442 results from forming curved drain fingerelectrode end 426 and the surrounding gate region 409 with increasedradii of curvature (r1) 436, r2 (441), r3 (471), and r4 (481) such thatr1<r2<r4. In addition, both r3 and r4 radii of curvature are optimizedto have a smooth curvature with no sharp discontinuities where they meetdrain finger electrode 420 and gate region 409 surrounding drain fingerelectrode 420, respectively.

Gate-to-drain spacing (d1) 467 is shown as being less than gate-to-drainspacing (d2) 457. As discussed above by reference to FIG. 3B, by formingthe gate-to-drain spacing such that d1<d2, in combination with theellipsoidal shape of curved drain finger electrode end 426, and thesmooth curvatures created by radii of curvature r1, r2, r3, and r4,allows for the formation of a more robust drain finger electrodetermination.

In some implementations, it may be advantageous for the ratio ofincreased width 446 to width 454 to be approximately equal to 2:1. Inother implementations, it may be advantageous or desirable for the ratioof increased width 446 to width 454 to be greater than 2:1. An advantageof using an ellipsoidal shape for curved drain finger electrode end 426is that such a shape can aid in minimizing the peak electric fields thatcan otherwise be present at the transition from a main body to a roundedsemicircular end termination region of a conventionally configured drainfinger electrode, such as semicircular drain finger electrode ends 125,in FIG. 1.

It is noted that width 454, increased width 446, and curved drain fingerelectrode end 426 are exaggerated in FIG. 4, for conceptual clarity.Thus, drain finger electrode 420 is not drawn to scale. Nevertheless,drain finger electrode 420 corresponds to drain finger electrodes 220 inFIG. 2, as well as to drain finger electrode 320 in FIGS. 3A and 3B, asnoted above.

In some implementations, further improvements in high current carryingtransistors can be realized with higher breakdown voltages and lowerON-resistance through use of tapered drain and source finger electrodes,which may be implemented in combination with a curved drain fingerelectrode end having an increased radius of curvature, corresponding tocurved drain finger electrode ends 226/326/426 in respective FIGS. 2,3B, and 4. Such tapered source and drain finger designs are described inU.S. Pat. No. 7,417,257, entitled “III-Nitride Device with ImprovedLayout Geometry,” and issued on Aug. 26, 2008. The entire disclosure ofU.S. Pat. No. 7,417,257 is hereby incorporated fully by reference intothe present application.

FIG. 5 presents a top plan view of an exemplary drain terminationincluding a tapered drain finger electrode enabling increased breakdownvoltage, in accordance with an implementation of the present disclosure.FIG. 5 shows gate region 509 surrounding tapered drain finger electrode520. Tapered drain finger electrode 520 has width 554, and includes atapered drain finger electrode main body 524 having width 534. Inaddition, tapered drain finger electrode 520 includes curved drainfinger electrode end 526 with increased radius of curvature (r1) 536relative to the radius of curvature of semicircular drain fingerelectrode ends 125 in transistor 100 of FIG. 1. That is to say, incontrast to transistor 100 having semicircular drain finger ends 125,radius of curvature (r1) 536 of curved drain finger electrode end 526 isgreater than a radius of curvature of a circle having a diameter equalto width 554.

As further shown in FIG. 5, width 534 of tapered drain finger electrodemain body 524 is larger than width 554. In other words, in contrast todrain finger electrode 320/420 for which width 334/434 of drain fingerelectrode main bodies 324/424 may be substantially equal to width354/454, tapered drain finger electrode 520 has a tapered drain fingerelectrode beginning width 534 that is greater than width 554.

A further approach to minimizing the termination peak electric fields isdescribed below with respect to FIGS. 6A, 6B, and 6C. FIG. 6A presents atop plan view of an exemplary transistor having a drain terminationenabling increased breakdown voltage, in accordance with animplementation of the present disclosure. The portion of transistor 600shown in FIG. 6A can correspond to the portion of transistor 200 labeledwith bracket 6A in FIG. 2. FIGS. 6B and 6C present cross-sectional viewsof exemplary transistors, in accordance with an implementation of thepresent disclosure. The cross-sectional views in FIGS. 6B and 6C cancorrespond to cross-section 6-6 in FIG. 6A in different implementationsof the present disclosure.

Transistor 600 includes drain finger electrodes 620 and source fingerelectrode 610 corresponding respectively to drain finger electrodes 220and source finger electrode 210 in FIG. 2. Transistor 600 also includesgate region 609 corresponding to gate region 209 in FIG. 2.

Similar to transistor 200, drain finger electrodes 620 areinterdigitated with source finger electrodes 610. Drain fingerelectrodes 620 and source finger electrodes 610 are situated in, on, orabove (or a combination thereof, i.e., non-coplaner) semiconductorsubstrate 672. Semiconductor substrate 672 can be, for example, aIII-Nitride material stack, such as a HEMT material stack, silicon,and/or other semiconductor material. Current conduction paths, such ascurrent conduction path 670, are in semiconductor substrate 672 betweendrain finger electrodes 620 and source finger electrodes 610. Gateregion 609 is configured to control current conduction in semiconductorsubstrate 672 through the current conduction paths between drain fingerelectrodes 620 and source finger electrodes 610.

Also in transistor 600, drain finger electrodes 620 have drain fingerelectrode main bodies 624 and drain finger electrode ends 626 (e.g.curved drain finger electrode ends 626) corresponding respectively todrain finger electrode main bodies 224 and drain finger electrode ends226 in FIG. 2. Source finger electrodes 610 have source finger electrodemain bodies 614 and source finger electrode ends (not shown in FIG. 6A)corresponding respectively to source finger electrode main bodies 214and source finger electrode ends 216 in FIG. 2. In some implementations,drain finger electrode main bodies 624 are coplanar with drain fingerelectrode ends 626 as described by FIG. 6B. In other implementations,drain finger electrode main body 624 is non-coplanar with at least aportion of drain finger electrode end 626 as described by FIG. 6C.

Referring to FIG. 6B, drain finger electrode 620 a is shown having drainfinger electrode main body 624 a and drain finger electrode end 626 a.Drain finger electrode 620 a is situated on semiconductor substrate 672.In particular, drain finger electrode main body 624 a and drain fingerelectrode end 626 a are both situated on semiconductor substrate 672 andare coplanar. Any or all of drain fingers electrodes 620 in transistor600 can have a similar cross-section as shown in FIG. 6B. However, byconfiguring any or all of drain fingers electrodes 620 in accordancewith the implementation shown in FIG. 6C, termination electric fields,which may develop during operation, particularly at drain fingerelectrode ends 626, can be further eased.

Referring now to FIG. 6C, drain finger electrode 620 b is shown havingdrain finger electrode main body 624 b and drain finger electrode end626 b. Drain finger electrode 620 b is situated on semiconductorsubstrate 672. In particular, drain finger electrode main body 624 b issituated on semiconductor substrate 672. However, dielectric material674 is situated between at least portion of drain finger electrode end626 b and semiconductor substrate 672. Drain finger electrode end 626 bis formed over dielectric material 674 and over semiconductor substrate672 such that at least a portion of drain finger electrode end 626 b isnon-coplanar with drain finger electrode main body 624 b. Furthermore,dielectric material 674 insulates drain finger electrode end 626 b fromsemiconductor substrate 672 thereby further easing termination electricfields, which may develop at drain finger electrode end 626 b. Thus,transistor 600 can have an even higher breakdown voltage and a loweron-resistance when using the drain finger termination topology shown inFIG. 6C.

Examples of dielectric material 674 include one or more dielectrics,such as any combination of silicon nitride, silicon oxide, oxynitride,or other insulating material. In certain implementations, dielectricmaterial 674 includes tetraethyl orthosilicate (TEOS). The dielectricsmay be provided in layers, such as in a dielectric stack, or may beprovided by a combination of similar or dissimilar materials.

In some implementations, dielectric material 674 is of a substantiallyuniform thickness. However, in the implementation shown in FIG. 6C,dielectric material 674 is an increasing thickness dielectric material.By configuring the thickness of dielectric material 674, the easing ofthe termination electric fields can be controlled, for example, bygradually increasing the insulation thickness along the length of drainfinger electrode end 626 b between drain finger electrode end 626 b andsemiconductor substrate 672.

Dielectric material 674 may be situated at least partially insemiconductor substrate 672. For example, dielectric material 674 may besituated in a trench in semiconductor substrate 672. However, in thepresent implementation, dielectric material 674 is situated on and oversemiconductor substrate 672. In doing so, drain finger electrode end 626b has increased elevation 676 and is non-coplanar relative to drainfinger electrode main body 624 b. Increased elevation 676 increases withthe increasing thickness of dielectric material 674.

Thus, FIGS. 6A and 6B present another approach, which can be utilized toenhance performance of a semiconductor device, such as a transistor or adiode. As such, this approach may be applied to a device such astransistor 100 in FIG. 1 without utilizing other approaches describedherein. Alternatively, any the aforementioned approaches can beemployed. For example, as shown in FIG. 6A, any of drain fingerelectrodes ends 626 can have an increased radius of curvature.Furthermore, any of drain finger electrode ends 626 can extend beyondany of source finger electrode main bodies 614, as shown. Also, any ofdrain finger electrode ends 626 can be a tapered drain finger electrode(not shown in FIG. 6A).

In accordance with various implementations disclosed by the presentapplication, termination electric fields at drain finger electrode endscan be reduced. As a result, disclosed implementations achieve anincreased breakdown voltage. The implementations disclosed herein canachieve the increased breakdown voltage while concurrently maintaining alow specific ON-resistance (on-resistance in the active area) through ashorter source-to-drain half pitch.

From the above description it is manifested that various techniques canbe used for implementing the concepts described in the presentapplication without departing from the scope of those concepts.Moreover, while the concepts have been described with specific referenceto certain implementations, a person of ordinary skill in the art wouldrecognize that changes can be made in form and detail without departingfrom the scope of those concepts. As such, the described implementationsare to be considered in all respects as illustrative and notrestrictive. It should also be understood that the present applicationis not limited to the particular implementations described above, butmany rearrangements, modifications, and substitutions are possiblewithout departing from the scope of the present disclosure.

1. A transistor comprising: drain finger electrodes interdigitated withsource finger electrodes; current conduction paths in a semiconductorsubstrate between said drain finger electrodes and said source fingerelectrodes; at least one of said drain finger electrodes having a drainfinger electrode main body and a drain finger electrode end, wherein atleast a portion of said drain finger end is non-coplanar with said drainfinger electrode main body.
 2. The transistor of claim 1, wherein adielectric material is situated between said drain finger electrode endand said semiconductor substrate.
 3. The transistor of claim 2, whereinsaid dielectric material elevates said drain finger electrode end oversaid semiconductor substrate.
 4. The transistor of claim 1, wherein saiddrain finger electrode main body is situated on said semiconductorsubstrate.
 5. The transistor of claim 1, wherein said drain fingerelectrode end has an increased radius of curvature.
 6. The transistor ofclaim 1, wherein at least one of said source finger electrodes has asource finger electrode main body coupled to a source pad, said drainfinger electrode end extending beyond said source finger electrode mainbody.
 7. The transistor of claim 1, wherein said drain finger electrodesare longer than said source finger electrodes.
 8. The transistor ofclaim 1, wherein said transistor is a high-electron-mobility transistor(HEMT).
 9. The transistor of claim 1, wherein said transistor comprisesa group III-V transistor.
 10. A transistor comprising: drain fingerelectrodes and source finger electrodes situated on a semiconductorsubstrate; said drain finger electrodes interdigitated with sourcefinger electrodes; a gate region configured to control currentconduction in said semiconductor substrate between said drain fingerelectrodes and said source finger electrodes; at least one of said drainfinger electrodes having a drain finger electrode main body and a drainfinger electrode end having an increased elevation relative to saiddrain finger electrode main body.
 11. The transistor of claim 10,wherein a dielectric material provides said increased elevation of saiddrain finger electrode end relative to said drain finger electrode mainbody.
 12. The transistor of claim 10, wherein said drain fingerelectrode end has an increased radius of curvature.
 13. The transistorof claim 10, wherein at least one of said source finger electrodes has asource finger electrode main body coupled to a source pad, said drainfinger electrode end extending beyond said source finger electrode mainbody.
 14. The transistor of claim 10, wherein said drain fingerelectrodes are longer than said source finger electrodes.
 15. Thetransistor of claim 10, wherein said drain finger electrode end has anincreased width relative to a width of said drain finger electrode mainbody.
 16. A transistor comprising: drain finger electrodesinterdigitated with source finger electrodes; a gate region configuredto control current conduction in a semiconductor substrate between saiddrain finger electrodes and said source finger electrodes; at least oneof said drain finger electrodes having a drain finger electrode end anda drain finger electrode main body, wherein at least a portion of saiddrain finger end is non-coplanar with said drain finger electrode mainbody; wherein an increasing thickness dielectric material is situatedbetween at least said portion of said drain finger electrode end andsaid semiconductor substrate.
 17. The transistor of claim 16, whereinsaid increasing thickness dielectric material elevates said drain fingerelectrode end over said semiconductor substrate.
 18. The transistor ofclaim 16, wherein said drain finger electrode end is substantially anellipsoid.
 19. The transistor of claim 16, wherein said drain fingerelectrode is a tapered drain finger electrode.
 20. The transistor ofclaim 16, wherein said transistor is a high-electron-mobility transistor(HEMT).